1. Field of the Invention
The present invention relates to a light irradiation apparatus, a crystallization apparatus, a crystallization method and a device, and relates to, e.g., a technique which generates a crystallized semiconductor film by irradiating a non-single-crystal semiconductor film with a laser beam having a predetermined light intensity distribution.
2. Description of the Related Art
A thin-film transistor (TFT) used for a switching element or the like which selects a display pixel in, e.g., a liquid crystal display (LCD) has been conventionally formed by using amorphous silicon or polysilicon.
Polysilicon has a higher mobility of electrons or holes than does amorphous silicon. Therefore, when a transistor is formed by using polysilicon, the switching speed and hence display response speed become higher than those in the case of forming the same by using amorphous silicon. Further, a peripheral LSI can comprise a thin-film transistor. Furthermore, there is an advantage of reducing the design margin of any other component. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at higher speed.
Since polysilicon comprises an aggregation of a plurality of crystal grains, when, e.g., a TFT transistor is formed, a crystal grain boundary or boundaries unfavorably present in a channel region, the crystal grain boundary serves as a barrier, and a mobility of electrons or holes is reduced as compared with that of single-crystal silicon. Additionally, each of many thin-film transistors formed by using polysilicon has a different number of crystal grain boundaries formed in a channel portion, and this difference becomes irregularities, resulting in a problem of unevenness in display in case of a liquid crystal display. Thus, there has been recently proposed a crystallization method which generates crystallized silicon with a large particle size enabling at least one channel region to be formed in order to improve the mobility of electrons or holes and reduce irregularities in number of crystal grain boundaries in a channel portion.
As this type of crystallization method, there has been conventionally known a phase-control excimer laser annealing (ELA) method which generates a crystallized semiconductor film by irradiating a non-single-crystal semiconductor film (a polycrystal semiconductor film or a non-single-crystal semiconductor film) with an excimer laser beam through a phase shifter approximated in parallel with the non-single-semiconductor film. Details of the phase-control ELA method are disclosed in, e.g., Journal of The Surface Science Society of Japan, Vol. 21, No. 5, pp. 278-287, 2000.
In the phase-control ELA method, a non-single-crystal semiconductor film is irradiated with a laser beam which has on an irradiation surface of the non-single-crystal semiconductor film a light intensity distribution having at least one inverse peak pattern (a pattern in which a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery) in which a light intensity at a point corresponding to a phase shift portion or line of a phase shifter is lower than that in the periphery. As a result, a temperature gradient is generated in a molten area in accordance with a light intensity distribution in an irradiation target area, a crystal nucleus or nuclei are formed at a part or parts which are solidified first or a part which is not melted in accordance with a point where the light intensity is minimum, and a crystal grows from the crystal nucleus in a lateral direction toward the periphery (which will be referred to as a “lateral growth” hereinafter), thereby generating a single-crystal grain with a large particle size.
Further, “Arrays of Large Si Grains Grown at Room Temperature for x-Si TFTs” by M. Jyumonji, et al., SID 04 Digest, pp. 434, 2004 discloses that positioning of a growth start point of a crystal is performed by irradiating a non-single-crystal semiconductor film with a light beam having a light intensity distribution with an inverse peak shape generated by a phase step of a phase shifter.
In the technique disclosed in this reference, a light intensity distribution with an inverse peak shape is formed by multiple phase steps obtained by dividing a phase of 360° into n steps. At this time, the phase difference per step is 360/n°. Furthermore, this reference describes that a light intensity at a bottom peak (an inverse peak point) in the light intensity distribution with the inverse peak shape can be adjusted by appropriately setting the number n of steps and appropriately setting the phase difference per step. In the multiple phase steps, the phase difference per step is decreased and the light intensity at the bottom peak becomes shallow (large) as the step number n is increased. Furthermore, the light intensity at the bottom peak can be adjusted to be close to a crystal growth start intensity by selecting a step having an appropriate phase difference.
FIGS. 24A to 24C are views illustrating simulation results of light intensity distributions obtained by a phase shifter 100 with two phase steps which has a phase area 100a of zero and a phase area 100b of 90°, a phase difference between these phase areas being 90°. When the two phase steps having the phase difference of 90° are used, a light intensity distribution with an inverse peak shape formed at a focus position of an image formation optical system is symmetrical with respect to each peak line 101 (a vertical line which is indicated by a broken line and runs through an inverse peak point of the light intensity distribution at the focus position) corresponding to each phase shift line 100c as shown in FIG. 24B. Conversely, as shown in FIGS. 24A and 24C, at defocus positions slightly shifted in the vertical direction from the focus position of the image formation optical system, the symmetry of the light intensity distribution with the inverse peak shape to be formed (the symmetry with respect to the peak line 101) largely collapses, and a position of each bottom beam is shifted (moved) in the lateral direction.
In general, when the two phase steps having a phase difference smaller than 180° is used, the symmetry of the light intensity distribution largely collapses at the defocus position distanced in the vertical direction from the focus position. Moreover, since the collapse of the symmetry is inverted depending on the defocus direction between the light intensity distribution shown in FIG. 24A and the light intensity distribution shown in FIG. 24C, a depth of focus becomes shallow (narrow). A shift direction of the bottom peak is on a phase advance side of the phase step (a portion depressed from the lower side in the figure) at the defocus position apart from the image formation optical system, and it is on a phase delay side of the phase step (a portion protruding toward the lower side in the figure) at the defocus position close to the image formation optical system. A board thickness deviation which can be a factor of defocus unavoidably exists in a processed substrate held at the focus position of the image formation optical system. That is, since there is a board thickness distribution on a given level on a glass substrate which is used for, e.g., a liquid crystal display, such defocus cannot be avoided.
The term “phase” pertaining to the present invention will be defined as follows, with reference to FIG. 26.
Consider the wavefront of an incident plane wave, which lies immediately behind a phase shifter. That part of the wavefront, which shifts in the propagation direction of light, is defined as “phase-advancing” side region. That part of the wavefront, which shifts toward the light source, is defined as “phase-delaying” side region. As FIG. 26 shows, the phase shifter has a protruding or thick part and a depressing or thin part on one surface. These parts border each other at a stepped portion. The protracting or projecting part is at the phase-advancing side region, and the depressing or receding part is at the phase-delaying side region.
This definition of phase can be applied also to other phase shifters that have neither a projecting part or a receding part. The phase may be controlled by using a fine pattern having lower resolution than the focusing optical system used. In this case, it suffices to apply the same definition of phase to the wavefront formed in the imaging field. For any phase shifter, the phase has a positive value if it advances. For example, +90° mans a phase advance, and −90° a phase delay.
Since a phase advance surface and a phase delay surface are alternately repeated in the two phase steps, the shift directions of the bottom peaks are alternately inverted. As a result, bottom peak positions are provided at irregular intervals, and hence crystal growth start positions (crystal grain positions) are also provided at irregular intervals, resulting in irregular shapes and sizes of crystal grains. Additionally, in general, of peaks provided on the both sides of the bottom peak, the light intensity of the peak on one side is raised and increased whilst the light intensity of the peak on the other side is lowered and decreased by defocus. As a result, the rising peak comes into contact with another rising peak, while the falling peak comes into contact with another falling peak, and a change in light intensity is amplified by the synergetic effect. This phenomenon becomes prominent as a pitch of the phase steps becomes small, and it is further emphasized when the light intensity distribution is converted into a temperature.
FIGS. 25A to 25C are views illustrating simulation results of light intensity distributions obtained by a phase shifter 110 with four phase steps whose phase difference is 90°. When the four phase steps with a phase difference of 90° is used, a light intensity distribution with an inverse peak shape formed at a focus position of the image formation optical system is symmetrical with respect to each peak line 101 (a vertical line which is indicated by a broken line and runs through an inverse peak point of the light intensity distribution at the focus position) corresponding to each phase shift line 110c as shown in FIG. 25B. Conversely, at a defocus position slightly shifted in the vertical direction from the focus position of the image formation optical system, as shown in FIGS. 25A and 25C, the symmetry of the light intensity distribution with the inverse peak shape to be formed greatly collapses, and a position of a bottom peak is shifted.
However, in general, in the case of the multiple phase step, since the phase advance directions are aligned in the same direction (a direction from the left side to the right side in the drawing) as different from the case of the two phase steps, the bottom peak shift directions are also aligned in the same direction (a direction from the right side to the left side in the drawing) (in this example, it is considered that a defocus quantity, i.e., a board thickness distribution of glass is locally fixed). Therefore, in the multiple phase step, the bottom peak positions are provided at equal intervals, and hence the crystal growth start positions (the crystal grain positions) are also provided at equal intervals, resulting in uniform shapes and sizes of crystal grains. Further, since the peak whose light intensity is raised and increased is in contact with the peak whose light intensity is lowered and decreased, a change in light intensity is offset (canceled out). Furthermore, considering that the light intensity distribution is converted into a temperature, changes in light intensity can be further homogenized.
As described above, although the multiple phase steps are advantageous as compared with the two phase steps, but there are the following problems. Firstly, in case of the multiple phase steps, a phase step having a phase difference of 360/n° alone can be realized. Specifically, in case of four phase steps shown in FIG. 25A, when an attempt is made to equalize phase differences of all the steps, a phase difference when returning to the zero-order step from the third step must be equal to the phase difference between other steps. Considering this restriction, 360/n° is the only phase difference in the multiple phase steps. Therefore, even though a phase difference between discrete phase differences (angles) determined as 360/n° under various conditions is optimum, this phase difference cannot be realized.
Secondly, in the case of multiple phase steps, a light modulation element (a phase shifter) is hard to be manufactured. That is, although manufacture of a phase step is generally realized by repeating irregular processing with a fixed depth more than once, the phase steps can be realized by performing processing for m times until the step number reaches 2m. Specifically, four (4=22) phase steps can be realized by performing processing for two times and five to eight (8=23) phase steps can be realized by performing processing for three times. In this manner, irregular processing for three times or more is necessary in order to realize five or more phase steps. Considering the difficulty in reprocessing over the once processed irregular surface and the difficulty in alignment (positioning), four or fewer phase steps which require irregular processing for two times can be realized, but it is very difficult to produce multiple phase steps which require irregular processing for three times or more. Even though, e.g., approximately six phase steps are desirable in order to set a light intensity distribution of the bottom peak to an appropriate intensity, the realization is difficult since irregular processing must be carried out for three times.